Project Sign was an official U.S. government study of unidentified flying objects (UFOs) undertaken by the United States Air Force and active for most of 1948.

Project Sign's final report, published in early 1949, stated that while some UFOs appeared to represent actual aircraft there was not enough data to determine their origin. However, prior to this final report, Sign officially argued that UFOs were likely of extraterrestrial origin, and most of the project's personnel came to favor the extraterrestrial hypothesis before this opinion was rejected and in 1948 project sign was reorganized and renamed Project Grudge, which was later renamed Project Blue Book in 1952.